Note that I have moved the shipping milestone to M121, and
have created a TAG review issue.
On Thu, Nov 2, 2023 at 10:07 AM Stephen Chenney
<schen...@chromium.org> wrote:
On Wed, Nov 1, 2023 at 11:11 AM Stephen Chenney
<schen...@chromium.org> wrote:
Answers inline. Regarding Ander's comment on the
current base_feature setting: I'll fix that.
Cheers,
Stephen.
On Wed, Nov 1, 2023 at 4:17 AM Yoav Weiss
<yoavwe...@chromium.org> wrote:
On Tue, Oct 31, 2023 at 1:45 PM Stephen Chenney
<schen...@chromium.org> wrote:
Contact emails
schen...@chromium.org
Specification
https://drafts.csswg.org/css-pseudo-4/#highlight-cascade
Summary
With CSS Highlight Inheritance, the CSS
Highlight pseudo classes, such as
::selection and ::highlight, inherit their
properties through the pseudo highlight
chain, rather than the element chain. The
result is a more intuitive model for
inheritance of properties in highlights.
Specifically, "When any supported property
is not given a value by the cascade ... its
specified value is determined by inheritance
from the corresponding highlight
pseudo-element of its originating element’s
parent element."
(https://drafts.csswg.org/css-pseudo-4/#highlight-cascade)
Blink component
Blink>CSS
<https://bugs.chromium.org/p/chromium/issues/list?q=component:Blink%3ECSS>
TAG review
None
Features are exempt from a TAG review only when
another vendor has already shipped them. That
doesn't seem to be the case here.
The feature is in the CSS pseudos spec and has been
for quite a while. The CSS Working Group has been
discussing issues too and Safari, at least, is
implementing according to the spec. I think the ship
has sailed for any major revision to the behavior.
(For context, there was no feature defined for this
change until recently because it was originally
viewed as a "make the code implement the spec" change.)
TAG Review Issue:
https://github.com/w3ctag/design-reviews/issues/914
TAG review status
Not applicable
Risks
Interoperability and Compatibility
The feature is still under implementation in
other browser engines, but the standards are
well developed and there is general
agreement on the spec. I think compat risk
is very limited at this time.
Does this feature change the behavior of
existing uses of ::highlight and ::selection? Is
there risk from breakage there?
The change aligns the behavior of ::selection with
Firefox. ::highlight is already implemented with
this behavior. There was breakage of ::selection due
to custom property handling, which was resolved at
the spec level and fixed in chromium before sending
this intent. No other bugs have come in over the
extended period that this has been on for
experimental web platform features (since M111).
My above comment is wrong: The spec defining this
feature does change the behavior of ::selection (not
::highlight) for all browsers. But evidence suggests
that the mitigations that sites used to work around the
old behavior still work with the new behavior, so
breakage is very limited. There was a single source of
significant breakage when the feature was first turned
on and the spec and code have been changed to maintain
the former behavior (related to custom properties on
root applying to ::selection). We have had zero other
reported bugs from enabling the feature experimentally.
Some history ... The spec was changed in response to an
issue where Firefox wanted to un-prefix -moz-selection
but was not obeying the old spec. As I understand it,
the selection style was checking for ::selection on the
selected element, using it if found, otherwise using the
root selection styling. All browsers were doing this.
Sites were designed to work around this through the
judicious use of ::selection on various elements. That
is, put ::selection on anything you wanted a specified
selection on, and if the inheritance was wrong, add a
more specific ::selection selector. Hence the heavy use
of custom properties to keep all these ::selection
declarations in sync. You can see this, for example, on
GitHub where they define ::selection for an element,
element > span, and element > span > span, and again for
light and dark mode. Sass even has an operator with a
comment on it saying they would remove it if the spec is
fixed.
This intent to ship does not break sites that have taken
this approach. Specific selectors for ::selection will
resolve to the same properties as they do now. The
improvement is that "element > span::selection" and
"element > span > span::selection" can now be removed in
favor of just "element::selection".
/Gecko/: Neutral
(https://github.com/mozilla/standards-positions/issues/548)
emilio@ is active in standards discussions
on the issues, but I am not aware of
implementation.
https://bugzilla.mozilla.org/show_bug.cgi?id=1703963
https://bugzilla.mozilla.org/show_bug.cgi?id=1703961
/WebKit/: In development
(https://github.com/WebKit/standards-positions/issues/95)
I have a private email from the Apple
engineer tasked with implementing. I don't
want to reveal PI.
/Web developers/: Developer ergonomics is
the primary concern motivating highlight
inheritance. See
https://github.com/w3c/csswg-drafts/issues/2474 for
the initial spec discussion related to the
behavior of ::selection. See
https://bugs.chromium.org/p/chromium/issues/detail?id=1490471 for
an example of a user seeing unexpected
behavior without this feature.
/Other signals/:
Ergonomics
None.
Activation
No. This reflects the already active
behavior for ::selection in Firefox and the
already used behavior for ::highlight,
::spelling and ::grammar.
Security
There are no security risks.
WebView application risks
None
Debuggability
Devtools supports highlight pseudos and
correctly shows the inheritance chain.
Will this feature be supported on
all six Blink platforms (Windows,
Mac, Linux, Chrome OS, Android, and
Android WebView)?
Yes
There are no cross-platform issues with
implementation and no reason to discriminate
on platform.
Is this feature fully tested by
web-platform-tests
<https://chromium.googlesource.com/chromium/src/+/main/docs/testing/web_platform_tests.md>?
Yes
https://wpt.fyi/results/css/css-pseudo?label=experimental&label=master&aligned
<https://wpt.fyi/results/css/css-pseudo?label=experimental&label=master&aligned>
highlight-cascade-* covers this
functionality. There are additional WPT that
make use of the feature in
https://wpt.fyi/results/css/css-highlight-api?label=experimental&label=master&aligned
<https://wpt.fyi/results/css/css-highlight-api?label=experimental&label=master&aligned>
Flag name on chrome://flags
experimental-web-platform-features
Finch feature name
HighlightInheritance
Non-finch justification
The feature was enabled as experimental way
back in M111 and stayed that way until M116
when it was switched back to test, and it is
back on experimental for M118. Developers
have significant experience with the feature
enabled via experimental web platform
features. There is no value to running a
finch trial given the large amount of
existing experience with the feature.
Requires code in //chrome?
False
Estimated milestones
Shipping on desktop 120
DevTrial on desktop 118
Shipping on Android 120
DevTrial on Android 118
Shipping on WebView 120
Anticipated spec changes
None
Link to entry on the Chrome Platform
Status
https://chromestatus.com/feature/5090853643354112
Links to previous Intent discussions
Ready for Trial:
https://groups.google.com/a/chromium.org/g/blink-dev/c/BbvI5VAguvk
This intent message was generated by Chrome
Platform Status <https://chromestatus.com/>.
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